Overview of defect characterization on GaN stacks for vertical device fabrication on 200 mm engineered QST® substrates

Abstract

The increasing demand for high-voltage (>1200 V) power devices has driven the epitaxy of thick GaN buffers on top of mechanically robust substrates, such as engineered QST® substrates. This study investigates the material crystalline defects in vertical GaN buffers grown via metalorganic chemical vapor deposition on 200 mm QST®, with and without pre-grown epitaxial lateral overgrown (ELOG) buffers. We present a systematic analysis of the threading dislocation density (TDD) using several destructive and non-destructive techniques such as transmission electron microscopy (TEM), cathodoluminescence (CL), electron channeling contrast imaging (ECCI), x-ray diffraction (XRD) and scanning probe microscopy (SPM). They reveal different dislocation types, i.e. edge, mixed and screw TDs. TEM reveals a ∼5 times TDD reduction for stacks employing ELOG buffers. ECCI, CL and XRD complement one another with ∼2–2.5 times reduced TDD on ELOG buffer. Whereas CL reveals all TDs involved in non-radiative recombination, ambiguity remains in ascribing the specific TD type. ECCI primarily identifies mixed TDs. XRD analyses of GaN(0002) and (10–12) reflections reveal a systematic improvement of crystal quality with increasing drift layer (DL) thickness. ELOG buffer reveals ∼4 times stronger screw TD reduction and >6 times stronger edge TD reduction with increasing DL thickness, compared to the one without. Conductive atomic force microscopy measurements show that only ∼0.1%–0.2% of the total TDs are electrically active contributing to vertical leakage, with >3 times reduction of such TDs in the ELOG buffer. Cross-sectional SPM, CL and ECCI imaging further validate the reduced defectivity in ELOG-enabled stacks. The scanning capacitance microscope (SCM) |dC/dV| maps show reduced amplitude at defect sites and CL imaging clearly visualizes these darker non-radiative recombination regions, whereas ECCI complements these observations. In summary, this study presents a comprehensive defectivity analysis of GaN-on-QST® substrates with ELOG buffers, demonstrating their potential for vertical GaN device fabrication and up-scaling.